Television transmitting system



p 17, 1940- J. D. M GEE ET AL- TELEVISION TRANSMITTING SYSTEM Filed Jan. 7, 1959 I nnnn 1.1?

INVENTORS JA MES DWYER MC GEE HANS GERHARD LUBSZYNSKI BY FRAN BiYTHEN A b NEY Patented Sept. 17, 1940 UNITED STATES PATENT OFFICE TELEVISION TRANSMITTING SYSTEM James Dwyer McGee, Ealing, London, Hans Gerhard Lubszynski, Hillingdon, and Frank Blythen, Hayes, England, assignors to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application January 7, 1939, Serial No. 249,744

In Great Britain January 11, 1938 2 Claims. (01. 178-72) This invention relates to television transmittelevision transmitting apparatus is provided ting apparatus of the kind in which a picture reemploying a picture resolving device of the kind solving device is employed having a substantially in which an image of the object for transmission continuous photo-sensitive surface upon which is projected onto a substantially continuous V? 5 an optical image is projected and in which an photo-sensitive surface, the electron image ob- '5 electron image derived from said surface is emtained therefrom being utilised so as to derive ployed for obtaining positive signals, and is picture signals therefrom, and wherein means mainly concerned with the re-insertion of lost are provided for obtaining from said photo- D. C., or low frequency components hereinafter sensitive surface a current or voltage represental0 referred to as D. C. component. tive of the average brightness of the optical im- In such television transmission apparatus varages projected onto said photo-sensitive surface ious proposals have heretofore been made for the and for indicating the value of said current or re-insertion of the D. C. component and in one voltage, 'thus enabling said surface to be cmof these proposals it has been suggested to obployed for D. C. reinsertion purposes or for intain the D. C. component by measuring the averdicating the optimum aperture for the optical age brightness of the picture projected onto the lens of the resolving device. transmitting tube by providing a photo-electric According to a further feature of the invencell upon which the image is also projected, the tion a television transmitting apparatus is proc'urrent derived from the photo-cell being em- 'vided employing a picture resolving device of ployed for D. C. re-i-nserting purposes. the kind in which an image of the object for 99 In some forms of picture resolving devices the transmission is projected onto a substantially image of the object is projected onto a substancontinuous photo-sensitive surface, the electron tially continuous photo-sensitive surface or cathimage obtained therefrom being utilised so as ode and the photo-electron image emanating to derive picture signals therefrom, the D. C.,

from the cathode is projected and focussed onto component of said signals being lost, and where- 25 a mosaic screen which acquires electrostatic in said photo-sensitive surface is connected in charges according to the intensity of the eleccircuit in such a way that a current or voltage tron image projected thereon, the elements of representative of the average brightness of the the mosaic screen being periodically restored to optical image projected onto said surface can be M a datum value by scanning with a cathode ray obtained, and means for feeding said current beam. In another form of picture resolving deor voltage after amplification, if required, 'to said vice the image is projected onto a similar photosignals so as to re-insert the lost D. C. composensitive cathode and the electron image emnents. anating from the cathode as a whole is scanned In order that the said invention may be clearly a over an apertured electrode, this type of transunderstood and readily carried into effect the mitting tube being called an image dissector. same will now be more fully described with ref- The D. C. component is usually lost at the reerence to the accompanying drawing which ilsolving device and various methods are employed lustrates a circuit arrangement for obtaining a or have been proposed for re-inserting the com signal suitable for D. C. re-insertion purposes. 40 ponent at various points in the amplifier chain. As shown, there is employed a picture resolv- 40 The D. C. component may require to be inserted ing device in the form of a cathode ray tube of at several points prior to transmission owing known type comprising an evacuated envelope l to the difficulty of constructing amplifiers capahaving a substantially continuous photo-electrode ble of handling very high as well as D. C. and cathode 2 onto which an image for transmission 5 low frequency signals. The present invention is projected by an optical lens 3. The electron is mainly concerned with the re-insertion of the image emanating from the cathode 2 is focussed D. C. component in transmitting apparatus emby means, not shown, and accelerated by an ploying resolving devices of the types described anode 4 onto a mosaic screen 5 associated with or other kinds in which the optical image is the signal plate 6. On the projection of the elecprojected on a photo-sensitive surface and has tron image onto the mosaic screen 5 the latter 50 for its main object to provide an improved appaacquires electrostatic charges which are periodiratus in which the photo-sensitive surface of the cally restored to a datum potential by scanning resolving device is employed for D. C. re-inserwith a cathode ray beam generated by a cathode tion purposes. ray gun I. The signal plate 6 is connected to According to one feature of the invention a a resistance 8 and on scanning the mosaic screen 55 picture signals are developed across the resistance 8 and are fed to a suitable amplifier prior to transmission. It will be found that the signals developed across the resistance 8 do not contain their D. C. component and according to the invention a signal suitable for re-inserting the lost D. C. component is derived from the photosensitive cathode 2. The cathode 2, as shown, is connected through a resistance 9, in turn connected to the negative pole of a source of E. M. F. the positive pole of which is earthed and connected to the accelerating electrode 4. The passage of the photo-electric current through the resistance 9 generates a voltage which is applied to the control electrode of a valve l 0, which.

is preferably a triode, having a high amplification factor, the anode of the valve being connected through a resistance II to the positive terminal which is earthed, as stated above. The cathode of the valve In is connected through a biassing resistance l2 to the negative pole of the abovementioned source, the resistance [2 also serving to provide negative feedback for straightening the characteristic of the valve. The photo-electric current is thus amplified by the valve I0 and appears as a negative potential across the resistance II and can be applied in any suitable way to the amplified picture signals derived from the resistance 8. For example, the potential developed across the resistance H may serve directly as a bias potential for the amplifier of the picture signals.

The present invention may be conveniently employed in conjunction with the invention described in the specification of co-pending patent application No. 35,791/37 in which the average brightness of the object is employed in conjunction with the average voltage at the output of a D. C. amplifier for D. C. re-insertion purposes the electrical difference of the two voltages being applied as a bias control to the input of the amplifier for the purpose of maintaining the D. C. level of the output signals at the required value.

In some cases the photo-electric current of the cathode 2 may be employed as an indication as to the amount of bias necessary to re-introduce the lost D. C. component. For this purpose a simple current meter may be connected in series with the cathode 2 or, alternatively, where a voltage indication is required a suitable voltmeter may be connected across the resistance 9. The bias control of the amplifier may thus be manually adjusted according to the current or voltage indication on the meter associated with the cathode 2.

In some cases it may be desired to obtain in a suitable manner a measure of the largest picture amplitude occurring and to employ this measure in conjunction with the average brightness of the object, so that the resultant indication is a combined contrast and D. C. indication.

The current or voltage indication obtained from the photo-sensitive cathode ray may also be employed to indicate the optimum aperture required to be used with the optical lens 3 through which the image is projected onto the photosensitive cathode.

We claim:

1. Television transmitting apparatus comprising a transmitting tube having a conducting lightresponsive electrode positioned at one end thereof, means for focusing an optical image upon said light-responsive electrode, a mosaic electrode and. a signal plate positioned parallel to the light responsive electrode at the other end of the tube,

potential means for accelerating and focusing the,

photo-electrons liberated from the light-responsive electrodeonto the mosaic electrode when an optical image is projected on thelight-responsive electrode, means in said tube adapted to produce a beam of electrons for scanning said mosaic electrode to produce picture signals at the signal plate, a discharge tube including a cathode, a control electrode and an anode, means for connecting the light-responsive electrode to the control electrode of said discharge tube, and means for maintaining said anode positive with respect to said cathode whereby the low frequency and direct current potential variations of the picture signals may be derived from said discharge tube.

2. Television transmitting apparatus comprising a transmitting tube having a conducting light-responsive electrode positioned therein upon which an optical image is adapted to be projected, a mosaic electrode and a signal plate also positioned in the tube in a plane parallel to said lightresponsive electrode, means for accelerating and focusing the liberated photo-electrons from the light-responsive electrode onto the mosaic electrode, means adapted to scan said mosaic by an electron beam to produce picture signals at the Y signal plate, an output circuit connected to the signal plate, a discharge tube including a cathode, a control electrode and an anode, means for connecting said light-responsive electrode to the control electrode of said discharge tube, and

means for maintaining said anode positive with respect to said cathode whereby low frequency and direct current potential variations may be derived from said discharge tube.

JAMES DWYER McGEE.

HANS GERHARD LUBSZYNSKI.

FRANK BLYTHEN. 

